Gday Damien
The EDID reader program gave me the clue.
I mentioned above that when the screen was working, device mangler was reporting the screen ID as an SEC5044,
but it lied :-)
Ref the dump from the EDID reader.
The three marked with the # are what was being reported.
When activated by plugging in the external monitor, the "active" internal monitor now became an NEC61BE, which translates to an NEC Multisync FP1375X (Analog)
Note!, all other models ( which includes data for all the external screens i have used to date ) list 3 times, but the NEC only had 1 listing???
Ignoring that, i then found a driver for the screen and went to device mangler and simply reset all 3 instances to the NEC driver, and viola, it all works.
Many thanks for the breadcrumb trail.
Andrew